1. Field of the Invention
This invention relates to compositions based on polyols and high molecular-weight diisocyanates with a low content of monomeric diisocyanates, and to their production and their use as binders for reactive one- or two-pack adhesives/sealants, reactive hot melt adhesives or solvent-based polyurethane adhesives.
Reactive polyurethane adhesives/sealants, particularly one-pack, moisture-curing systems, generally contain polymers that are liquid at room temperature with urethane groups, optionally urea groups and reactive isocyanate groups. For many applications, these compositions are solvent-free and very highly viscous and/or paste-like, and they are processed at room temperature or at a slightly elevated temperature of between about 50° C. and about 100° C.
2. Description of the Related Art
Reactive, one-pack, moisture-curing polyurethane hot melt adhesives are moisture-curing or moisture-crosslinking adhesives that are solid at room temperature and are applied as an adhesive in the form of their melt, and the polymer components of which contain urethane groups and reactive isocyanate groups. As a result of the cooling of this melt after application and joining of the substrate parts to be bonded, a rapid physical setting of the hot-melt adhesive first takes place by means of its solidification. This is followed by a chemical reaction of the isocyanate groups still present with moisture from the environment to form a crosslinked, infusible adhesive. Reactive hot-melt adhesives based on isocyanate-terminated polyurethane prepolymers are described e.g. by H. F. Huber and H. Müller in “Shaping Reactive Hotmelts Using LMW Copolyesters”, Adhesives Age, November 1987, pages 32 to 35.
Laminating adhesives can either be constituted similarly to the reactive hot melt adhesives or they are applied as one-pack systems from a solution in organic solvents; a further embodiment consists of two-pack, solvent-based or solvent-free systems in which the polymer constituents of the one component contain urethane groups and reactive isocyanate groups and, in the two-pack systems, the second component contains polymers or oligomers with hydroxyl groups, amino groups, epoxy groups and/or carboxyl groups. In these two-pack systems the isocyanate group-containing component and the second component are mixed immediately before application, usually with the aid of a mixing and dispensing system.
Reactive polyurethane adhesives/sealants are distinguished by a very high overall performance. In the last few years, therefore, more and more new applications have been opened up for these adhesives/sealants. Compositions for such adhesives and/or sealants are already known from very many patent applications and other publications.
In addition to many advantages, these polyurethane compositions also have some disadvantages caused by the system. One of the most serious disadvantages is the residual monomer content of isocyanates, especially the more volatile diisocyanates. Adhesives/sealants, and especially the hot melt adhesives, are processed at an elevated temperature. The hot melt adhesives, for example, are processed at between 100° C. and 200° C., laminating adhesives between room temperature and 150° C. Even at room temperature, volatile isocyanates such as TDI or IPDI have a not insignificant vapour pressure. This noticeable vapour pressure is particularly serious especially in the case of spray application, since in this case significant quantities of isocyanate vapors can occur over the object being treated, which are toxic owing to their irritant and sensitising action. Protective measures must therefore be taken to prevent health hazards to persons charged with the processing. These measures, such as e.g. the requirement to monitor compliance with the maximum allowable concentration, are expensive. In particular, exhaust ventilation measures for the vapours at the point of formation and delivery are very cost-intensive and also form an obstacle to certain application methods, such as in particular the spray application of reactive polyurethane adhesives/sealants.
The development of reactive polyurethane compositions with a drastically reduced proportion of monomeric diisocyanates is therefore highly desirable for these fields of application, as in some cases this would make it possible to use them in many applications in which their use was not previously possible owing to the workshop hygiene problems mentioned above.
According to the Schulz-Flory statistical model, when diisocyanates with isocyanate groups of approximately equal reactivity are reacted with hydroxyl group-containing compounds, the remaining content of monomeric diisocyanate in the reaction product depends on the NCO/OH ratio of the reactants in the prepolymer synthesis. With an NCO/OH ratio of 2, as is frequently necessary for the prepolymer composition, approximately 25% of the monomeric diisocyanate used remains as a monomer in the prepolymer. If, for example, 10 wt. % diphenylmethane diisocyanate (MDI) is used in a prepolymer synthesis with an NCO/OH ratio of 2, an order of magnitude of approximately 2 wt. % monomeric MDI is found in the prepolymer in accordance with the statistical estimate mentioned above. At 150° C. the pure MDI already has a vapour pressure of 0.8 mbar. While it is true that, according to Raoult's law, this vapour pressure is lower in compositions, it is still higher than the non-toxic range for workshop hygiene. Under the application conditions described above, especially when being applied as a hot melt adhesive over a large area in a thin layer, considerable quantities of the residual monomer therefore pass into the air space above and must be removed by exhaust ventilation. A significant reduction in the monomer content by a power of ten, by reducing the NCO/OH ratio, cannot generally be achieved in practice since the average molecular weight would then increase exponentially and the resulting polyurethane compositions would become extremely viscous and would be impossible to process. In practice, therefore, other routes are taken for prepolymer synthesis. For example, synthesis is performed with a sufficiently high NCO/OH ratio and the monomeric diisocyanate removed in a second step after pre-polymerisation; this can take place by distilling off the unreacted monomeric diisocyanate under vacuum, for example, or by subsequent chemical bonding of the monomeric diisocyanate. Thus, EP-A-316738 describes a process for the production of polyisocyanates containing urethane groups with a urethane group-free starting diisocyanate of no more than 0.4 wt. % by reacting aromatic diisocyanates with polyhydric alcohols and subsequently removing the unreacted, excess starting diisocyanate, the removal of the excess starting diisocyanate by distillation taking place in the presence of an aliphatic polyisocyanate containing isocyanate groups.
EP-A-0393903 describes a process for the production of prepolymers in which, in a first step, monomeric diisocyanate is reacted with a polyol. A catalyst is then added in a sufficient quantity for a considerable proportion of the residual isocyanate functionality to be converted to allophanate functionality. When the theoretical NCO content is reached, the reaction is stopped by rapid cooling and addition of salicylic acid.
WO-95/06124 describes polyurethane compositions with a low proportion of monomeric diisocyanates which are produced by reacting polyols with trifunctional isocyanates and optionally adding monofunctional chain terminators. A disadvantage of this process is the low availability of trifunctional, low molecular-weight isocyanates; the trifunctional homologues of diphenylmethane diisocyanate in particular are not commercially available in pure form.
In the work by V. A. Kudishina and E. F. Morgunova, Sin. Fiz. -Khim. Polim. (1970), part 7, 125–129, cold-curing polyurethane adhesives based on hydroxyfunctional polyesters or polyethers and isocyanate-containing hardeners are described. The isocyanate-containing hardeners are reaction products of tolylene diisocyanate (TDI) or of diphenylmethane diisocyanate (MDI), ethylene glycol and glycerin. It is stated that these hardener components lead to a substantial reduction in the toxicity of the adhesives, although these still produce a quantity of isocyanate of 1.7% in the air space of the corresponding test chamber. These workplace concentrations of isocyanates are no longer tolerable in industrialised western countries under today's standards of workshop hygiene.
Despite the aforementioned prior art, therefore, there is still a need for improved polyurethane compositions with a low proportion of monomeric diisocyanates that are suitable for use as adhesives/sealants, especially for reactive hot melt adhesives. In particular, the raw materials used for these should be readily and cheaply accessible and readily converted, and the adhesion properties should be at least equal to those of the conventional hot melt adhesives.